26.5 Telescopes

Telescopes are our windows to the cosmos, allowing us to peer into the depths of space. They work by gathering and focusing light from distant objects, using lenses or mirrors to magnify and reveal celestial wonders.

Telescopes come in various types, each designed to observe different parts of the electromagnetic spectrum. From visible light to radio waves and X-rays, these instruments help us uncover the Universe's secrets across multiple wavelengths.

Telescopes

Magnification and light gathering

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• Telescopes utilize lenses or mirrors to collect and focus light from distant objects
  • Objective lens or primary mirror gathers light and forms an image (Hubble Space Telescope's 2.4-meter primary mirror)
  • Eyepiece lens magnifies the image formed by the objective lens or primary mirror (10mm eyepiece)
• Magnification is calculated by dividing the focal length of the objective lens by the focal length of the eyepiece lens
  • $[Magnification](https://www.fiveableKeyTerm:Magnification) = \frac{f_{objective}}{f_{eyepiece}}$
  • Longer focal length objective and shorter focal length eyepiece produce higher magnification (f_objective = 1000mm, f_eyepiece = 10mm, magnification = 100x)
• Light-gathering power is determined by the area of the objective lens or primary mirror
  • Larger diameter objective or mirror collects more light (8-inch vs 4-inch telescope)
  • Light-gathering power is proportional to the square of the diameter (16 times more light for twice the diameter)
  • The diameter of the objective lens or primary mirror is also known as the aperture
• Resolving power measures the ability to distinguish between two closely spaced objects
  • Depends on the diameter of the objective lens or primary mirror and the wavelength of light (larger diameter and shorter wavelength improve resolving power)
  • Larger diameter and shorter wavelength result in better resolving power (Hubble Space Telescope's 2.4-meter mirror and visible light)
  • Also referred to as angular resolution

Refracting vs reflecting telescopes

• Refracting telescopes employ lenses to collect and focus light
  • Objective lens is located at the front of the telescope tube
  • Light passes through the objective lens and is focused to form an image
  • Eyepiece lens magnifies the image formed by the objective lens
  • Advantages: good image quality, sealed tube protects optics (Galileo's telescopes)
  • Disadvantages: limited size due to lens weight and chromatic aberration (largest refractors around 1 meter in diameter)
• Reflecting telescopes use mirrors to collect and focus light
  • Primary mirror is positioned at the back of the telescope tube
  • Light reflects off the primary mirror and is focused to form an image
  • Secondary mirror directs the light to the eyepiece or camera (Newtonian reflector)
  • Advantages: larger mirrors possible, no chromatic aberration, cheaper to manufacture (Keck telescopes with 10-meter mirrors)
  • Disadvantages: open tube allows air currents and dust to affect image quality (requires regular cleaning and maintenance)

Telescopes across electromagnetic spectrum

• Telescopes can be designed to observe different wavelengths of the electromagnetic spectrum
  • Optical telescopes detect visible light with wavelengths between 380 and 700 nm (Hubble Space Telescope)
  • Radio telescopes detect radio waves with wavelengths longer than 1 mm
    • Often use large dish antennas to collect and focus radio waves (Arecibo Observatory's 305-meter dish)
  • Infrared telescopes detect infrared radiation with wavelengths between 700 nm and 1 mm
    • Require cooled detectors to minimize thermal noise (James Webb Space Telescope)
  • Ultraviolet, X-ray, and gamma-ray telescopes detect high-energy photons
    • Must be placed above Earth's atmosphere, which absorbs these wavelengths (Chandra X-ray Observatory)
    • Use grazing incidence mirrors or coded aperture masks to focus high-energy photons (NuSTAR X-ray telescope)
• Multi-wavelength astronomy offers a more comprehensive understanding of celestial objects
  • Different wavelengths reveal different physical processes and properties (radio waves trace cold gas, X-rays trace hot gas)
  • Combining observations from various parts of the electromagnetic spectrum helps create a more complete picture of the Universe (studying a galaxy in radio, infrared, visible, ultraviolet, and X-ray wavelengths)

Advanced Telescope Technologies

• Interferometry combines signals from multiple telescopes to achieve higher resolution
  • Allows for effective apertures much larger than individual telescopes (Very Large Array)
• Adaptive optics systems correct for atmospheric distortions in real-time
  • Improves image quality for ground-based telescopes (Keck Observatory)
• Modern telescopes use charge-coupled devices (CCDs) for digital imaging
  • Provides higher sensitivity and easier data processing compared to photographic plates
• Spectroscopy allows telescopes to analyze the chemical composition of celestial objects
  • Breaks light into its component wavelengths to study emission and absorption lines
• The diffraction limit sets the theoretical maximum resolution for a telescope
  • Determined by the wavelength of light and the aperture size